1. Technical Field
The invention relates to broadcasting of television programs providing a high definition image of 1250 lines over broadcasting channels having insufficient band-width to covey all pixels of all successive video fields. This situation requires image processing techniques reducing the volume of data to be transmitted. In general, the image processing techniques comprise spatio-temporal subsampling and movement compensation involving the calculation of a motion vector field during the analysis of the image prior to broadcasting, the transmission of assistance data containing the vector field at the time of broadcasting and finally, when decoding in a receiver for displaying a high definition image, the use of the assistance data to reconstruct the image.
One of the problems associated with this method of broadcasting is the simultaneous existence of high definition receivers and of first generation receivers which must both deliver a good quality image. Now, time subsampling, particularly with field skip or frame skip, leads to jerking of the image restored by a first generation receiver which cannot use the assistance data to overcome it.
2. Prior Art
Although the problem is general in nature, reference will be made especially in what follows to the broadcasting of television programs by the HD-MAC system defined by the European project EUREKA EU95. The transmission of a high definition image comprising 1250 lines of 1440 points would require if effected using a multiplex of digital data in base band and a chrominance and luminance analog signal, a bandwidth very much greater than that offered by satellite broadcasting channels. To reduce the flow of data required, use will be made:
for low temporal activity pictures areas, of purely temporal subsampling, spreading the spatial data of one or more source frame analysed over several successive transmissions fields; PA0 for high temporal activity picture areas, of wider space subsampling or non-linear processing of the signal, by motion compensation, which results in not respecting the 50 Hz analysis of the source pictures and implies the use of assistance data transmitted so that the HD decoder can reconstruct the image; PA0 for pictures having a very high time activity (amplitude of the vector greater than the swing of the motion estimation) or for picture areas with so called "erratic" motion, spatial subsampling of each analysis field.
General information regarding HD-MAC may be obtained by reference to publications, particularly "Colloque TVHD 87", Ottawa, Oct. 4-8, 1987, Vol. 1, pp. 6.2.2.-6.2.28, P. Bernard et al: "Analyse de structures de sous-echantillonnage spatio-temporel d'un signal TVHD en vue de sa transmission dans un canal MAC"; publications WO-A-8 705 770(British Broadcasting Corp.) and WO-A-8 801 462 (British Broadcasting Corp.). The contents of such documents are included in the present specification by way of reference.
In what follows, particular attention will be directed to a situation in which the source pictures (or source picture portions) with high temporal activity are broadcast by spreading out over two successive broadcasting fields the alternate lines of the same field (odd video field) of the video source picture. At reception, the contents of the two transmission fields are used for restoring a same odd video field of the high definition frame and the missing video fields (even fields) are restored from the available video fields (uneven fields) and from assistance data transmitted separately and giving an indication concerning the field of motion vectors.
However, a first generation receiver will strictly reproduce, in the form of two successive video fields, two transmission fields corresponding to the same HD scanning field (source picture at a given time), hence jerking of high temporal activity pictures since the analysis rythm will in fact be 25 Hz, i.e. 40 ms.
If it is attempted to substitute for the above subsampling technique, called "field skip", the transmission of one point out of four only of each of the successive frames (i.e. to carry out a wider space subsampling), the picture delivered by the HD-MAC receiver, which has no complete video field any longer, is considerably degraded.
The second transmission field of each pair cannot be formed by points either which are not the points of the source picture, but points offset so as to take into account the motion vector at the source point, for this method faces problems of zones with overlapping or uncovering which no longer respect bijectivity between the points to be transmitted and the actually transmitted points.